Added new chat start at and end at fields to admin model
Added model manager, start-at and end-at fields.

The model manager provides functionality to find upcoming live chats to
advertise on the home page and askmama page. It also provides a function to
return the currently in progress live chat, to allow redirecting the user to the
live chat directly from the AskMAMA link in the top navigation, and the
advertisement.
Removed the link to popular unanswered questions. We are not using jmbo-comments
The livechat view template uses the livechat inclusion tag to display the chat
Added a context processor that checks for a live chat that is in progress, and adds it to the request context.
Context processor now provides in progress live chat
Added a migration to add the chat starts and chat ends fields to the db table